For any sceptic of the continued capacity of science to uncover new truth, to pave the way for previously unimagined applications, there is hardly a better corrective than to invite reflection on recent discoveries in the science of light. It may be unscientific to say that light is unfathomable, but it certainly is a characteristic of the subject that there is always more to be learned, just when the utmost depths seem within grasp. There is no better illustration than the specific subject of the volume before you.

It has long been known that light conveys energy, and the associated linear momentum has also been understood since the days of Maxwell and Bartoli. With angular momentum the history is more recent, and the property a little less straightforward. What we quickly learned is that light has a propensity to convey angular momentum, depending on its state. The pioneering work in which Beth established a link with circular polarisation is nonetheless already three-quarters of a century old. Once the quantum theory of light was developed, many would have surmised that the science was complete, the concept of angular momentum so beautifully related to the unit spin of the photon-the hallmark of a boson. But what has been discovered in the past quarter century has shown that the spin angular momentum is only half the story – and the other half has no ending yet in sight.

Recent developments in the angular momentum of light are leading to new and wideranging applications, even as the subject presents fresh challenges to long established and cherished concepts.